Evolution and internal structure of the in the

Autor(en): Hänni, Reto / Pfiffner, O. Adrian

Objekttyp: Article

Zeitschrift: Eclogae Geologicae Helvetiae

Band (Jahr): 94 (2001)

Heft 2

PDF erstellt am: 11.10.2021

Persistenter Link: http://doi.org/10.5169/seals-168886

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http://www.e-periodica.ch 0012-9402/01/020161-11 $1.50 + 0.20/0 Eclogae geol. Helv. 94(2001) 161-171 Birkhäuser Verlag. Basel. 2001

Evolution and internal structure of the in the Bernese Oberland

Reto Hänni1-2 & O.Adrian Pfiffner1

Key words: -and-thrust structures, synsedimentary faulting. Helvetic nappes, western

ABSTRACT ZUSAMMENFASSUNG

During the last century, many detailed field studies have been carried out in Im vergangnenen Jahrhundert ist die lokale Geologie un Berner Oberland in the Bernese Oberland. Data from these studies were combined with new field /ahlreichen Arbeiten detailiert untersucht worden. Darauf aufbauend und dala to restore the Axen and the Drusherg nappes in three cross sections. The kombiniert mit neuen Felduntersuchungen wurden die Drusberg-Decke und criteria for validating retrodeformable cross sections were used to extend the die Axen-Decke in drei Querschnitten abgewickelt. Die Berücksichtigung limits of interpretation. Results show that synsedimentary normal faults played geometrischer Kriterien für abwickelbare Profile erlaubte dabei eine erhebliche a key role in the evolution of the nappes. Retrodeforming the nappes suggests Einschränkung des Interpretationsspielraums in erodierten Deckenteilen. that Cretaceous synsedimentary faults within the Drusberg had reactivated Es stellte sich heraus, dass kretazische synsedimentäre Normalbrüche in Jurassic faults within the Axen nappe. Alpine compression partly reactivated der Deckenkinematik eine Schlüsselrolle spielten. Derartige Brüche findet these faults once again but simultaneous!) developed coniugate reverse man in der Drusberg-Decke: die Deckenabwicklungen lassen vermuten, dass faults: subsequent folding and shearing lead lo Ihe hitherto enigmatic BQrgle- bei ihrer Entstehung jurassische Normalbrüche in der Unterlage, der heutigen Sylere structure. The Schilthorn thrust fault was found to be an important out- Axen-Decke. reaktiviert wurden. Während der alpinen Kompression wurden of-sequence ihrust. causing complex nappe internal structures such as Ihe lec- diese Brüche teilweise reaktiviert und führten in Kombination mit konjugierten tonic isolation of individual Ihrust slices. Ihe Kiental-phase folding of the Rückaufschiebungen und anschliessender Faltung und Scherung zu der Schilthorn fault clearly indicates a polyphase deformation hisiorv of Ihe Drusberg lokal gut bekannten Burgle-Sylere Struktur. Die Schilthorn-Überschiebung and the Axen nappes. isl eine bedeutende out-of-sequence Überschiebung und führte zu komplexen deckeninternen Strukturen in Form tektonisch vollständig isolierter Schuppen. Die Verfaltung der Schilthorn-Überschiebung während der Kiental- Deformationsphase zeugt von der mehrphasigen Verformungsgeschichte der Drusberg- und der Axen-Decke.

Introduction

The Helvetic zone of Central Switzerland and the Bernese Mesozoic-Cenozoic cover (e.g. Doldenhorn nappe). Further Oberland is composed of two nappe complexes separated by a north, this complex contains allochthonous cover sheets (Sub- major thrust fault with several tens of kilometers of displacement. alpine and Subalpine Molasse). This major thrust fault is the basal thrust ofthe Helvetic The sediments of the Helvetic zone were deposited on the nappes proper. The Helvetic nappes are characterized by a European margin of Tethys. In the course ofthe Alpine orogeny complex interplay of folds and faults involving Mesozoic- these sediments were thrust and transported to the north Cenozoic cover rocks. The Axen nappe (Fig. 1) contains and northwest. As a general rule, the paleogeographically essentially Jurassic sediments and is overlain by the Drusberg southern Helvetic units experienced a larger amount of nappe, which is built of Cretaceous-Cenozoic strata (Pfiffner displacement than the northern ones, which resulted in the 1993). The units in the footwall of the basal thrust of the development of an antiformal stack on top of the Aar massif in Helvetic nappes are referred to as . To the the transect ofthe Bernese Oberland (Fig. 1). The Doldenhorn south, this Infrahelvetic complex is a thick-skinned fold-and- nappe contains recumbent folds with a crystalline core forming thrust belt involving pre-Triassic crystalline rocks part of to the Aar massif. Across the Lauterbrunnen Valley to (Aar massif) and their autochthonous-parautochthonous the east, the separation between the Mesozoic cover and the

1 Geologisches Institut. Universität Bern. Baltzerstrasse 1. CH-3012 Bern. Switzerland. E-mail: [email protected] 2 now at Geotechnisches Institut AG. Seestrasse 22. 3700 Spiez. Switzerland. E-mail: [email protected]

The Helvetic nappes in the Bernese Oberland 161 •—r ~n Section II Violasse Section Mi tri hern Aar massif Doldenhorn nappe Southern Aar massif Gellihorn nappe 5nj Wildhorn nappe «e Axen nappe <*>. Drusberg nappe Unterhasl Subhehetic unn Ultrahlevetie and nappes Nappe boundary

Thrust fault (Ax.-D Section I H

t i Normal fault

10 km Lauterbrunnen •: a Q 1 Menta - Q Fig. 1. Tectonic sketch map of the Bernese Oberland. Thick lines are traces of cross sections shown on plate I.

crystalline core diminishes and the Doldenhorn nappe passes Helvetic nappes is missing in the Bernese Oberland owing to into the autochthonous-parautochthonous units typical of the erosion. In order to completely reconstruct the eroded and central and eastern Aar massif. subsurface parts of the nappes, results of former investigations From the Kander Valley to the west, the Gellihorn nappe had to be puzzled together. Fieldwork focussed on the upper separates the Wildhorn nappe from the underlying Doldenhorn parts of the Helvetic nappes, which are particularly important nappe. To the east, the Gellihorn nappe quickly diminishes for the tectonic interpretation of the eroded part of the nappes. in volume along strike until its complete disappearance Three balanced sections were constructed along with their east of the Kiental Valley. restored counterparts. The Jurassic and Cretaceous parts of the Wildhorn nappe The aim of this paper is to analyse the importance of ("Stockwerks") exhibit an increasing tectonic independence synsedimentary extensional structures for section retrodeformation going east, resulting in the eventual separation into two and to discuss their effect on the evolution of nappe distinct nappes which can be traced into eastern Switzerland (see internal structures in subsequent Alpine compression. also Pfiffner 1993). the Drusberg nappe (Cretaceous Stockwerk), and the Axen (Jurassic Stockwerk). nappe 2. Cross sections The Jurassic Stockwerk has been shown to contain a number of tight to nearly isoclinal folds (Arbenz 1922: Günzler-Seiffert 2.1. Construction method 1925; Hänni et al. 1997) cut by numerous tear faults (Pilloud 1990) and low angle reverse faults (Schilthorn. Männlichen). Three digitized sections across the study area were balanced The Cretaceous Stockwerk is disharmonically folded above and retrodeformed using suitable computer software such as the Jurassic Stockwerk owing to the thick, incompetent Palfris Geosec 4.0. Despite these powerful tools, section balancing marls separating the two Stockwerks (cf. Pfiffner 1993). As will and retrodeformation requires a lot of iterative work by hand. be shown later the two Stockwerks experienced a displacement Inevitably, the reconstruction of eroded structures and subsurface of up to 10 km by bedding parallel thrusting on this detachment extrapolation demands a certain amount of interpretation. horizon. However, section balancing allows testing of a profile's The Helvetic nappes are overlain by Ultrahelvetic and geometrical correctness by comparing volumes and line-lengths Penninic units of a more internal origin, which outcrop as erosional before and after deformation. In addition, a retrodeformable relics or "klippen" (Fig. 1). cross section not only has to be geometrically correct but it As a rough estimate, about half of the net volume of the also has to be kinematically plausible. Only the contradiction-

162 R. Hänni & O. A. Pfiffner Retrodeformed Drusberg and Axen nappes

Section I

Hnhguni-Sundljucnen fault Bachli-Giesenen faull

lvl\ Hm;jl l.tull Sic: (VI'.'

Section II

HnhiMiit Sundlauencn t.iult nhli ìiesenen t'jult

Drusberg nappe

\\en nappe

tabebera

Umd crystalline basement

Section III Bachli-Giesenen i.tuli

Burgle tau II

Quinten limestone

Hochstollen 1 m

______

F,'FL.L'LLL'L'L-L.L.L-L.L-L-L-F-F-F-F-F-F-F-F-F-L'L.L'.'F'F-F-F-F-'. ..'¦.'-.'..'..'..'..'..'..'.,'..'..'..'..'•.'..'•.'..'.

Fig. 2. Volume and line-length balanced restoration of the sections shown on plate I. The Axen and Drusberg nappes have been independently retrodeformed and correlated using the Bachli-Giesenen fault as a common feature. White dotted lines indicate the future basal thrusts of Ihc nappes. Note Ihe location of the "Lias von Steineben;" in section 1 and the vicinity of the Hohuant-Sundlauenen and the Aabebera fault in section II.

less return of a cross section to its undeformed state can nappes are entirely volume balanced and line-length guarantee a viable interpretation. Finally the cross sections were balanced with respect to the stratigraphie tops of the Glock- checked for consistency with the structural style observed in haus-Fm.. Ouinten limestone. Kieselkalk-Fm. and this segment of the Alpine orogen. Schrattenkalk-Fm. For completeness Triassic and lower Jurassic The sections were constructed using a field map at the beds were added to the retrodeformed section although the scale of 1:25 000 and numerous literature sources (see volumes of these strata are not accessible to direct observation. below). In addition, the results of the NRP 20 research project The Molasse in section II is drawn after Schlunegger et (Pfiffner et al. 1997) were used to constrain the Helvetic al. (1993) using heavy mineral boundaries as stratigraphie nappes within a large-scale tectonic framework. Each section markers. exhibits certain features that affect the construction and interpretation of the neighbouring section. Therefore, the 3D 2.2. Section 1 (plate I) structural geometry with its lateral changes has to be taken into account to achieve mutual consistency amongst the cross This easternmost section runs SSE along the right-hand side sections. The retrodeformed sections (Fig. 2) show the initial of the Kiental Valley (Fig. 1). The map by Krebs (1925), and state of the Drusberg and Axen nappes before deformation. the work by Liechti (1930). Zwahlen (1986) and Schürch In section II. the Drusberg and Axen nappes are entirely (1991 supplied valuable information for this section. The line-length and volume balanced. In sections I and III. these unbalanced geometry shown for the Doldenhorn nappe reflects

Ihe Helvetic nappes in the Bernese Oberland 163 NW SE

Bachli-Giesenen fault r sberg ppe

Kk

Border chain Fig. 3a. Enlargement ol section I. showing the present-day geometry of the Bachli-Giesenen growth laull (Kiental valley). T: Tertiarj Sk Schrattenkalk-Fm. Kk: Kieselkalk-Fm. Si Sichel Diphyoides limestone. P: Palfris-Fm. 0: Axen Quinten limestone, (i: (iloekhaus-Fm. For locatali! I km Infrahelvetic see plate I.

NW SE

Schilthorn fau Wid Andnst

Drusber nappe y Fig. 3b. Enlargement of section I. showing the internal structure of the Axen and Drusberg s* y Po nappes. The Schilthorn fault (doited line) offsets the hangingwall for about 2.5 km and isolates Sichel limestone at Wild Andrisl. Note the small lens of Jurassic at the roof thrust of the yy o° Upper Sk: (i ....-• lower Jurassic "Lias von Steineberg". Schrattenkalk-Fm. Kk: Kieselkalk-Fm. Si: Sichel/Diphy-

1 s m limestone. E: Er/egg-I m. H: Hochstollen-Fm. Ci: "Lias v^—Steineberg Glockhaus-Fm. For location see plate I.

new fieldwork that was partly carried out together with Jenni ern counterpart by the Bachli-Giesenen fault. This Mesozoic (1999). growth fault runs parallel to strike and outcrops in the Kiental Important lateral changes in nappe structure occur on Valley. On its southern hanging wall block, the strata end either side of section I. To the SW of the section, a lower unit discordantly on the horizontal basal thrust fault of the Drusberg ("Bundstock element". Zwahlen 1986) underlies the Wildhorn nappe, indicating a rotated hanging wall ramp (Fig. 3a). This nappe and consists of lower Jurassic to Teritary sediments that point will be discussed in more detail in section 4 indicate a more northerly origin for this unit. Along strike The southern part ofthe Axen nappe is cut by the relatively toward the east, the Gellihorn nappe wedges out and the young, gently dipping Schilthorn fault that causes a displacement Doldenhorn nappe loses its nappe character, grading into imbricate of the hanging wall of some 2.5 km to the northwest (Fig. thrusting within the Aar massif and its core (Infrahelvetic com- 3b). This fault, which was probably caused by early shortening pex). In section I. the more internal part of the Axen nappe of the Doldenhorn nappe, cuts across the Axen nappe and rests on the Doldenhorn nappe and was overprinted owing to merges into the Drusberg thrust, thereby increasing the shortening of the latter (Kiental phase. Günzler-Seiffert displacement of the latter. 1941b), whereas further east the Axen nappe rests on the In- Tectonically higher Ultrahelvetic and Penninic units of trahelvetic thrust sheets. Despite the differences in the footwall. more internal origin are locally wrapped beneath the basal individual folds within the Axen nappe are generally Helvetic thrust (Zwahlen 1993). The same is true for the quite continuous and can be traced along strike for distances of "Kiental Flysch" ("KF" in section I), a tectonic mélange at the 10 km and more. base of the Penninic Niesen nappe. These findings point to As mentioned by Liechti (1930). the northern part of the out-of-sequence thrusting (i.e. younger, more external thrusts Drusberg nappe ("Border Chain") is separated from its south¬ cut older thrusts of more internal origin).

164 R. Hänni & O. A. Pfiffner 2..?. Section ll (plate I) fault causes an imbrication of upper Jurasic limestones and is folded by a later deformation event (see section 7 and Fig. 6). The section line links the two wells Linden-1 and Thun-1 in a The exact geometry of the southernmost nappe parts (below straight line trending 345° and continues at 317 after a kink at "Lütschine") is uncertain. Normal faults on the inverse limb of Thun-1. the Middle Jurassic are projected from the east It is based on our own field data, maps by Günzler-Seiffert (Menkveld 1995). (1933. 1938). Collet & Paréjas (1928. 1931). Beck (1910). Within the Doldenhorn nappe, apart from the nappe-like Haldemann et al. (1980) and work by Pilloud (1990). Gold- "digitation supérieure" (Collet & Paréjas 1931). the sediments schmid (1926). Günzler-Seiffert & Wyss (1938). and Herb are not detached from their crystalline basement. As a result, (1980). the structures within the Doldenhorn nappe in section II can Owing to the wells Linden-1 (Maurer et al. 1978). Thun-1 be traced into the folded basement-cover contact of the (Micholet 1992). seismic lines (Vollmayr 1992) and sedimentological parautochthonous Aar massif. This is an important difference investigations (Schlunegger et al. 1993: Diem 1986). the compared to section I. where thick incompetent lower to middle structure of the Subalpine Molasse is quite well-known for this Jurassic marls form the core of the Doldenhorn nappe and section. The youngest sediments in the footwall of the Hilfern acted as a detachment horizon. thrust, the early Aquitanian Gitzischöpf Conglomerate (Schlunegger et al. 1996). indicate that the Hilfern thrust did 2.4. Section III (plate I) not reach the present-day surface before about 23 Ma. The Blueme thrust is drawn with a complete hanging wall to Section III establishes a structural link between the two sides illustrate the total thrust displacement. Supposedly, the front of the of the Unterhasli Valley. Because of the easterly dip of the Blueme thrust sheet was being eroded during its emplacement fold axes, information about tectonically lower parts must be by thrusting. The youngest sediments in the footwall of the taken from the west, whereas higher parts are projected from Blueme thrust sheet (15 Ma) pertain to the very diachronous the east. The section line runs from the subalpine Molasse to Gunten Quartzite Conglomerate of the Plateau Molasse. the crystalline basement in a 139° direction (Fig. 1). Unpublished Shortening of the Subalpine Molasse is estimated to be at least maps were at our disposal for profile construction (sheet 19 - 20 km. In cross section II (plate I), this amount of shortening Brienz. sheet Innertkirchen. Dräyer 1999). For the Drusberg is shown to be compensated by imbrications on the nappe we considered the work by Schider (1912) Michel northern flank of the allochthonous Aar massif. This solution (1921). Jost-Stauffer (1993) and Staeger (1944). for the Axen was chosen because it is difficult to merge the thrust displacements nappe Günzler-Seiffert (1925, 1934a, 1938. 1952). Günzler- from the Aar massif into the Triassic detachment horizon Seiffert & Müller (1934). Günzler-Seiffert & Wyss (1938), beneath the . On the other hand, the Staeger (1944). Dr. von Moos (1978). Kellerhals & Häfeli sedimentary cover of the Aar massif is hardly detached from the (1983). Pilloud (1990), Rowan (199.3). Menkveld (1995) and basement in the Lauterbrunnen Valley. However, there Hänni et al. (1997). Finally the deep part of the section relies remains the question where to take the additional 12 km (Laubscher on seismic data (Pfiffner et al. 1997). According to latest mapping 1965. Burkhard 1990) of shortening which are required by Schwizer (sheet Brienz. oral commun.), the second for the Jura mountains, a subject that is still under debate Quinten limestone above km 174 in the cross section ("Quinten (Burkhard 1999). We favour the idea of a detachment horizon limestone 5a" according to Günzler-Seiffert 1925) is that starts with relatively little displacement near the Aar massif. overturned. This new finding greatly improves the structural coherence The hangingwall of the detachment fault is thought to be across the Unterhasli Valley. The structure of the stiffened by thick Molasse sediments such that any increase in Infrahelvetic complex is drawn according to Furrer (1949). Halde- displacement along the detachment requires the footwall to be mann et al. (1980). Scabell (1926) and Kammer (1989). shortened (e.g. by partial inversion of Permo-Carboniferous In the southern part of the Axen nappe, the upper Jurassic grabens). Quinten limestone is detached from the middle Jurassic, forming Projection errors are minimal for the northern part of the a Stockwerk of its own within the Axen nappe ("Malm- Drusberg nappe ("Border Chain"). The Bachli-Giesenen fault stockwerk der Axen-Decke". Menkveld 1995). Analogous to below Lake Thun and the southern eroded part of the Drusberg section I and II. the internal part of the Axen nappe is cut by a nappe are projected into the section plane from the west. gently dipping fault that can be traced to the west (Fig. 1) and The Burgle and Sylere faults are located precisely on the which is therefore considered to be identical with the section line. In this section, the Gellihorn nappe is absent and the Schilthorn fault. In section III this fault runs close above the Axen nappe rests directly on the Infrahelvetic complex. topographic surface (to the north of "Rosenlaui" in section Compared to section I. the volume of middle Jurassic sediments is III). considerably larger. Unlike the interpretation by Louis (1924). Large parts of the Drusberg nappe are hidden in the the southern part of the Axen nappe (Männlichen) is cut by a subsurface and the correct geometry is difficult to assess (Jost- relatively young fault corresponding to the Schilthorn fault. Stauffer 1993: Michel 1921). As a consequence, there is a certain Before joining the basal thrust of the Drusberg nappe, this leeway in the interpretation ofthe Drusberg nappe. In the

The Helvetic nappes in the Bernese Oberland 165 Infrahelvetic complex of section III between Rosenlaui and reach their maximum thickness to the east of the Unterhasli Urbachsattel. a tectonic slice with large offset occurs along the Valley, thereby play an important role as detachment horizon. basal Helvetic thrust (Läsistock-Schuppe: Scabell 1926). Within the Axen nappe, the middle Jurassic Glockhaus-Fm. in particular exhibits significant lateral thickness changes across the Unterhasli valley. This variation in thickness is held to be 3. The Axen and the Drusberg nappes responsible for the development of distinctly different structures The Wildhorn nappe of the Wildhorn area, located 25 km to that are not easily correlated from one side of the valley the west of the study area, consists of a Jurassic and a to the other. Cretaceous-Tertiary Stockwerk. Going east, two nappes individualize: As shown in Fig. 2. the basal thrust of the Axen nappe- the Axen nappe containing the Jurassic beds and the Drusberg forms a flat at the base of the Glockhaus-Fm.. stepping over nappe containing the Cretaceous and Tertiary sediments. synsedimentary faults (e.g. Burgle fault). To the north, ihe As discussed by Pfiffner (1993), this change in structural style basal thrust forms a ramp through the entire Mesozoic is controlled by the mechanical stratigraphy at which incompetent sequence. This ramp may well correspond to a major shales of the Palfris-Fm. played an important role as a synsedimentary fault initiated in Early Jurassic times, separating the stratiform detachment horizon. The lateral emergence of these "Alemanic high" (Trümpy 1952) from a subsiding basin. This two nappes is rather continuous, which makes it difficult to fix high was approximately located at the site of the future Aar the eastern limit of the Wildhorn nappe. On the tectonic map massif. The sedimentary cover of the northern part of the Aar of Fig. 1. the eastern limit of the Wildhorn nappe is chosen to massif is mainly composed of carbonates and lacks shaly units follow the Bachli-Giesenen (Kandertal) fault. This interpretation of sufficient thickness to act as a detachment horizon, with the is favoured because the synsedimentary fault causes an result that this sedimentary sequence has remained attached abrupt lateral increase in Cretaceous sediment thickness to its crystalline basement. A noteable exception occurs at the toward the east and therefore an important change in structural western end of the Aar massif. Here, a basin of Early and Middle style (Zwahlen 1986). Stratigraphically. the Drusberg nappe Jurassic age accumulated shales interlayered with represents the Cretaceous cover of the Jurassic Axen limestones and sandstones which now form the Doldenhorn nappe. sediments.

3.2. The Drusberg nappe /. The Axen nappe The Drusberg nappe contains the lower Cretaceous and Teri- The Axen nappe contains a sedimentary sequence from lower tary sediments, detached along the shales and marls of the Jurassic to Berriasian. The shales of the lower Aalenian Mols- Berriasian Palfris-Fm. The lower Cretaceous sediments up to Fm. occur along the basal thrust and are restricted to the the Barremian consist of platform carbonates. Younger Cretaceous southernmost part of the Axen nappe. Their volume is difficult units (Garschella-. Seewer-. Amden- and Wang-Fm.) to assess owing to erosion. The main volume of middle Jurassic only occur in the SE part of the nappe where they have sediments consists of sandy limestones of the Glockhaus-Fm. escaped pre-Eocene erosion. The Bajocian (Hochstollen-Fm.) is made up of alternating The part of the Drusberg nappe to the north of the marls and limestones and increases in thickness toward the synsedimentary Bachli-Giesenen fault is also called the "Randkette" south. Bathonian and Callovian marls (Erzegg-Fm.) together or Border Chain. As a morphotectonic unit, the Border Chain with the Oxfordian marls (Schilt-Fm.) form the incompetent is a relatively coherent rigid plate compared to the rest of the footwall of the Quinten limestone (Kimmeridgian-Tithonian). Drusberg nappe. This can be explained by the relative lack of a thick massive limestone sequence. The roof thrust of the incompetent units separating the competent ones in the Border Axen nappe follows the Berriasian Palfris-Fm. and is identical Chain. In contrast, within the southern Drusberg nappe with the floor thrust ofthe Drusberg nappe. carbonates are interlayered with marls and here the nappe Many structures within the Axen nappe change their internal structure is characterized by folds. appearance quite quickly and it is often difficult to trace them There is a kinematic relationship between the Axen and laterally over large distances. The competent Quinten Drusberg nappes because of their simultaneous shortening and limestone is an important structural marker horizon and easily transport. As a general rule, thrust faults from the Axen nappe recognizable in the field. The thickness, of incompetent marls join the basal thrust of the Drusberg nappe, increasing the above and below the Quinten limestone generally increases displacement of the Drusberg nappe relative to the Axen nappe. toward the south and east, which leads to enhanced folding of the Quinten limestone within its incompetent host rock. A 4. The Bachli-Giesenen fault drastic increase in thickness of the incompetent middle Jurassic leads to disharmonie folding between the Quinten The Bachli-Giesenen fault separates the Border Chain from limestone and the Hochstollen-Fm. in the southern part of the the southern part ofthe Drusberg nappe. In detail, it is a complex Axen nappe from section III to the east (Menkveld 1995: fault system rather than one single fault (Zwahlen 1986). Hänni et al. 1997). The Callovian-Oxfordian marls, which It is well exposed in the Kiental Valley, where the strata ofthe

166 R. Hänni & O. A. Pfiffner NW SE :¦: Section Wetterlatte ¦<¦ ->¦ Section Höchst i NW SI i

S<.hrdtlenkjlk

FF QuilllfN linicstotii- Cretàceo« -Ni Pllfrâ marl Sv.hr.iiicnk.ik

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Future basjl Ihnisl ul the Drusberg nanne i^\ \ \ Quinmcrkalk \ J \ [Svnsalimcnlarv Bachi hr/egg Fm. I Giesenen laull

üocUwu- m Basement

Fig. 5. Sketch of the kinematic evolution of the Bilrgle-Sylere structure. The initial geometry is given by two synsedimentary faults: the Burgle fault and the Fig. 4. Schematic section across the Bachli-Giesenen growth fault before smaller, probably conjugate Sylere fault. In the course of Alpine shortening. Alpine overprint. To the south of Ihe fault, the thickness of the stratigraphie the competent Outnten limestone (light grey) between the two faults is sequence is increased by at least 640m. The upper Cretaceous and the Drus- squeezed and rotated into its present-day position. The Sylere fault was berg-Fm. are missing from the northern block. The future basal Ihrust of the inverted as a thrust fault and rotated into parallelism with the Burgle fault. The Drusberg nappe, indicated by the dashed line, must form a ramp to overcome Burgle fault itself was not or only partly reactivated as a thrust fault. the offset of the Berriasian Palfris marls. Incompetent units are shown in grey shading.

southern Drusberg nappe end discordantly against a subhorizontal, 5. The Burgle and Sylere faults gently dipping thrust surface (section I. plate I). The southern end of the Border Chain is bounded by a normal The Burgle and Sylere faults are two well-known faults in the fault that rejoins this thrust surface. The northern part of the Axen nappe (Stauffer 1920: Goldschmid 1926: Staender 1943; Drusberg nappe south of the Bachli-Giesenen fault shows a Günzler-Seiffert 19.33. 1934b. 1941a, 1944: Herb 1980: Pilloud ramp anticline, where steeply dipping and even overturned 1990: Rowan & Kligfield 1992: Rowan 1993). They both run strata abut the gently dipping Drusberg thrust. The restored parallel to strike, with the Burgle fault being the one further to geometry of this structure is shown in Fig. 4. The future (Drusberg) the north (Fig. 5). The two faults cross the lower Lauterbrunnen thrust fault formed a ramp at the site of the former Valley, where they form two spectacular, steeply south growth fault. Juxtaposition of the two flats led to the formation dipping normal faults. A hook-shaped piece of Quinten of a fault-bend anticline in the southern block and opened a V- limestone is clamped between the two faults. Until now. two shaped space between the Tertiary strata of the two blocks. contradictory theories about the evolution of this structure have This means that there is a structural discontinuity between the been proposed. Pilloud (1990) considered both faults to be Border Chain and the southern part of the Drusberg nappe initially parallel synsedimentary faults with approximately equal and. as Liechti (1930) pointed out. there exists no synclinal link displacements, which have been passively folded during nappe between the two. Comparing the sediments on each side of the shortening. In contrast. Rowan (1993) interpreted the structure Bachli-Giesenen fault, there is strong evidence for a to have evolved from two south vergent backthrusts that synsedimentary origin of this fault. subsequently rotated into their current subvertical orientation The total increase in sediment thickness caused by the due to a large amount of simple shear that affected the Axen Bachli-Giesenen growth fault is about 640 meters. To judge nappe as a whole. Neither explanation is in accord with our from differences in the Tertiary, the total vertical field data and we suggest an alternative kinematic scenario in displacement along this fault had to be even larger. The displacement the following section (see Fig. 5). on the Bachli-Giesenen fault is therefore far larger than As discussed in section 4. the Bachli-Giesenen fault was a the displacement on any other synsedimentary fault reported synsedimentary normal fault with a vertical displacement of from the Drusberg nappe (e.g. Liechti 1930: Colombi 1960). at least 640 m. affecting the Cretaceous and Tertiary strata of The Bachli-Giesenen fault is a feature common to both the the Drusberg and the Jurassic strata of the Axen nappe (Fig. Axen and the Drusberg nappe an therefore can be used to fix 4). We consider the Burgle fault to be the continuation ofthe the initial relative position ofthe two nappes. Considering the Bachli-Giesenen fault in the Axen nappe. Synsedimentary large displacement along the Bachli-Giesenen fault, its activity of the Burgle fault during the middle Jurassic was continuation within the Axen nappe must have had a comparable postulated by Günzler-Seiffert (1941a) and the present-day offset (see sections 5 and 6). geometry of the Burgle fault is readily explained as a reacti-

The Helvetic nappes in the Bernese Oberland 167 vated normal fault. It is plausible that the Burgle fault was the Drusberg nappe, and the Aabeberg fault of the Axen initiated during lower Jurassic rifting and was thereafter nappe. A synsedimentary origin of the Aabeberg fault was repeatedly active up into the Tertiary as a so-called "persistent postulated by Günzler-Seiffert (1941a). Within the errors of fault" (Günzler-Seiffert 1941a). The restored lengths of the our quantitative retrodeformation. it can be suggested that the top of the Quinten limestone and top of the Glockhaus-Fm. Aabeberg fault was the continuation of the Hohgant-Sundlauenen reveal the original geometry of the two faults, suggesting the fault. This possibility is supported by recent findings Sylere fault may be interpreted as a smaller, conjugate fault of speleologists (Häuselmann, oral commun.), which suggest to the Burgle fault (Fig. 2. section II. and Fig 5). The Sylere that the synsedimentary displacement of the Hohgant-Sundlauenen fault was subsequently inverted as a south vergent backthrust fault is significantly larger than the 50 to 100 m and then rotated into its present orientation. To explain the proposed by Colombi (1960). In this case, the Hohgant-Sundlauenen rotation, we consider the Quinten limestone as a competent fault should have a continuation within the Axen nappe, and discontinuous single layer embedded in less competent which we consider to be the Aabeberg fault. middle Jurassic and lower Cretaceous marls. We envisage a passive rotation under subhorizontal compression of the 7. The Schilthorn thrust fault competent Quinten limestone together with the Sylere fault. During this movement, the Burgle fault was not or only partly The Schilthorn thrust (Figs. 3b and 6) is a relatively gently inverted. We find that the simple shear model postulated by dipping thrust fault in the southern part of the Axen nappe with a Rowan & Kligfield (1992) does not explain the parallelism of thrust displacement of about 2.5 km. The type locality of this the two faults. thrust (Schilthorn) is located between sections I and II. Slices of Quinten limestone mark the thrust surface and indicate the repetition of thick Middle Jurassic Glockhaus limestones (the 6. The relative position of the retrodeformed Drusberg and cableway station "Birg" stands on such a Quinten limestone Axen nappes slice). The deep-water facies of the middle Jurassic in the In a first step, the Drusberg and the Axen nappes were hangingwall of the thrust underlines its southerly origin independently retrodeformed along the three cross sections. In a (Pilloud 1990). This southerly, internal facies led Günzler-Seiffert second step, the retrodeformed nappes were positioned relative (1934a) to the opinion that the upper block was derived from to each other in a way that the Bachli-Giesenen fault of the Ultrahelvetic realm. Pilloud (1990) called this block the the Drusberg nappe and the Burgle fault of the Axen nappe "Schilthorn element of the Axen nappe" tt) emphasize its merge into one single fault (Fig. 2). This correlation is consistent tectonic independence from the underlying Axen nappe. The with NW-directed thrusting along the Drusberg thrust, as thrust concept outlined by Rowan (1993) is worked out and indicated by field observations. In addition, the vertical displayed in the cross sections (Figures 3b and plate I). displacement of the Bachli-Giesenen fault of at least 640 m is in In section I of plate I the Schilthorn thrust cuts across the accordance with the displacement along the Burgle fault of middle and upper Jurassic and finally merges into the Palfris about 700 m. The reconstruction shown in Fig. 2 displays the marls, contributing to a further displacement of the Drusberg synsedimentary normal faults as running down into the pre- nappe. The piece of Sichelkalk that forms today"s Wild Triassic crystalline basement. This is in analogy with eastern Andrist (in section I) was plucked off from the base of the Switzerland were equivalent faults are associated with syn- Drusberg nappe and was then overridden by the Drusberg fault breccias containing components of Triassic dolomites nappe during motion on the Schilthorn thrust. The solution (Trümpy 1952). The retrodeformed section (Fig. 2) also presented in section II (plate I) brings together field answers an old local problem concerning the origin of the "Lias information from both sides of the Lauterbrunnen Valley. The von Steineberg" (Fig. 3b). These lower Jurassic strata are in Schilthorn thrust is shown to cut a recumbent syncline tectonic contact with their middle Jurassic hangingwall. as is consisting mainly of middle Jurassic sediments and produces a evident from the occurrence of a lens of upper Jurassic repetition of the Quinten limestone on the normal limb of the limestone between the lower and middle Jurassic strata (Künzi syncline. As shown in Fig. 6. the repeated Quinten limestone 1975). The lower Jurassic strata have therefore been taken to was subsequently folded, indicating a later compressive form an independent tectonic element by Zwahlen (1993). A deformation phase. The Schilthorn thrust cannot be traced much simpler explanation is offered by the retrodeformed continuously from section II towards the NE. Nevertheless, section of Fig. 2: The synsedimentary Bachli-Giesenen fault we can find a very similar thrust fault in section III (plate I), juxtaposed the "Lias von Steineberg" and the upper Jurassic (Quinten) which runs closely above the surface from "Rosenlaui" to the limestone of the southern block. Subsequent reactivation north. plucked off a piece of Quinten limestone and brought middle The Schilthorn thrust cuts the basal thrust of the Axen Jurassic strata on top of lower Jurassic strata. nappe as an out-of-sequence thrust (Fig. 3b). It probably originates Another interesting feature of the retrodeformed section from the southern part of the Doldenhorn nappe and is the resulting proximity of the Hohgant-Sundlauenen fault, was folded during the main shortening phase of the Doldenhorn which is a synsedimentary normal fault in the northern part of nappe (Kiental phase, see below).

168 R. Hänni & O. A. Pfiffner SI NW

Vhllth„r„ "'rusi __ f. 9. y Ï,^ ^ -' ZTD ::.: 7 F V Fig. 6. At the Spallenhorn (Saustal) the Quinten limestone is repeated by the Schilthorn fault, a r ¦¦ r ù fault cutting across earlier formed folds. The later folding of this thrust fault in the Saustal illustrates f the multi-phase deformation of the Axen nappe.

8. The deformation sequence (iii) The Grindelwald phase (Günzler-Seiffert 1941b) is de¬ The deformation sequence in the Helvetic nappes and the Aar fined by tilting of the basal thrust of the Axen nappe into a massif can be compared with a foreland propagating fold and subvertical position, best visible in sections II and III. In thrust system, where new thrust faults form more externally section I and to the west of it. large scale bending of the than the preceding ones. Superposing of individual deformation Kiental phase axial surfaces in the Doldenhorn nappe is events led to features like foliations, folds and folded ascribed to the Grindelwald phase (Burkhard 1988). thrust faults, which have been used for definition and relative dating of several defomation phases. One major problem when trying to correlate deformation (1) The Plaine Morte phase (Burkhard 1988) describes the phases from different regions is their regionally limited validity. emplacement of highly allochthonous thin sheets of sediments As discussed by Pfiffner et al. (1997), the Plaine Morte from the more internal and southerly. Ultrahelvetic phase can be correlated to the Pizol phase in eastern Switzerland, domain onto the future Helvetic nappes. and the Prabé phase to the Calanda phase, but the Kiental (2) The Prabé phase (Burkhard 1988) designates the shorten¬ phase, for instance, is by definition bound to the Doldenhorn ing and main transport phase of the Wildhorn nappe. During nappe. To the east of the Lauterbrunnen valley, where this event, the basal (Plaine Morte phase) thrust of the the Doldenhorn nappe no longer exists. Kiental phase structures Ultrahelvetic units was folded. cannot be identified. In a similar way, the Grindelwald (3) The basal thrusts of the Axen/Wildhorn and Gellihorn phase cannot easily be correlated into eastern Switzerland nappes were overprinted by later events. Depending on the (Pfiffneretal. 1997). locality, these events can be subdivided in up to three We try to defuse this situation by relating the locally individual deformation phases. defined deformation phases to three superordinate. orogen-scale (i) The Trubelstock phase (Burkhard 1988) is the earliest, but deformation events (Table 1 These are: it is restricted to the western Aar massif and seems to be of rather local character. The out-of-sequence Schilthorn 1. Emplacement of southern tectonic units on top of the future thrust, which was folded by subsequent Kiental phase folding, Helvetic nappes. In the Bernese Oberland this event is must have formed in a deformation phase similar to described by the Plaine Morte phase, in eastern Switzerland it the Trubelstock phase, although there is no direct structural is defined as the Pizol phase (Pfiffner 1977. 1978. Milnes & link to the locality where the Trubelstock phase is Pfiffner 1977). defined. 2. Shortening of the Helvetic nappes and their transport onto (ii) The Kiental phase (Günzler-Seiffert 1941b) is defined by the region of the future Aar massif. This event is described the emplacement of the Doldenhorn nappe, overprinting by the Prabé phase for the Bernese Oberland and by the the earlier thrust faults. In section I. which runs through Cavistrau and a part of the Calanda phase for eastern the locality where the Kiental phase is defined, continuous Switzerland (Pfiffner et al. 1997). axial planes indicate that the basal thrust of the Gellihorn 3. Shortening and uplift of the Aar massif. For the Bernese nappe was folded during the Kiental phase shortening of Oberland, the Grindelwald and the Kiental phases are the Doldenhorn nappe. attributed to this event. In fact, the Doldenhorn nappe pos-

The Helvetic nappes in the Bernese Oberland 169 ^Deformation 'elative sequence of events ^v events Emplacement of Shortening and transport of Continued transport of the Region \. southern units the Helvetic nappes onto Helvetic nappes and the future Aar massif shortening of the Aar massif Eastern end of 1 Pizol 1 ICavistraull Calanda | I Calanda || Ruchi | the Aar massif Haslttal- 1 Plaine Morte 1 1 Prabe I I Gnndolwaki | Lauterbrunnen Table 1. Local deformation phases can not easily [ Trubelstock Western end of be correlated over large distances along strike of ] 1 | Plaine Morte Prabé I ] 1 Kiental 1 the Aar massif the orogen. although the orogenic processes related 1 Grindelwald 1 to these phases are well comparable.

sesses a crystalline core (the southern Aar massif) from be considered as the continuation ofthe Bachli-Giesenen fault. which the sediments are more or less detached. The formation It turned out that a similar connection between the Hohgant- of the recumbent folds and thrust faults within the Sundlauenen fault and the Aabeberg fault is plausible. Doldenhorn nappe are Kiental phase structures. The Synsedimentary faults caused discontinuous changes in the mechanical deformation resistant Gastern granite, situated to the north stratigraphy within the nappes. In places, this resulted in of the Doldenhorn nappe's crystalline core, was overridden unusual structures, such as the Burgle and Sylere faults. In by the Doldenhorn nappe. Its late upwarp bent the overlying general, synsedimentary normal faults were passively folded structures and tilted the basal thrusts of the Helvetic rather than reactivated as reverse faults during nappe nappes, which is attributed to the Grindelwald phase. In compression. eastern Switzerland, deformation of the Aar massif is The Schilthorn thrust offsets the base of the Axen nappe attributed to the Calanda phase, which - in absolute time - by some 2.5 km as an important out-of-sequence thrust. It was later in the Infrahelvetic complex compared to the probably extends beyond the Haslital valley to the east. The overlying Helvetic nappes. Transport of the Helvetic Schilthorn thrust crosscuts the deformed Axen nappe and is nappes continued during shortening of the Aar massif as is itself folded. The deformation of the Axen nappe is therefore evident from the relation between penetrative foliations the result of several superimposed deformation events. above and beneath the basal thrust of the Helvetic nappes. Deformation phases defined by local overprinting relationships have The last phase of transport of the Helvetic nappes is correlated the disadvantage that they can not be traced over great to the Ruchi phase. distances. This problem can partly be overcome by relating local deformation phases to larger scale deformation events such as. In contrast to eastern Switzerland, where transport along the for example, the formation of the Aar massif dome. basal thrust of the Helvetic nappes continued during the shortening of the Aar massif, the Grindelwald phase shortening of the Acknowledgements western Aar massif led to enhanced rotation of the basal We acknowledge funding by the Swiss National Science Foundation (grant nr. Helvetic thrust, further this thrust making any transport along 20-43*246*95). The paper benefittet from comments by Martin Burkhard and impossible (Pfiffner et al. 1997). This continued shortening of the Neil Mancktelows Aar massif during the Grindelwald phase points to a late phase of deformation restricted to the western Aar massif, which might REFERENCES be linked to shortening and folding of the Jura Mountains. Arblnz. P. 1922: Die tektonische Stellung der grossen Doggermassen im Berner Oberland- Eclogae geol. Helv. XVII. 326-328. 9. Conclusions Beck. P. 1910: Geologische Karte der Gebirge nördlich von Interlaken. Spezialkarte Nr. 56a (1:50 000). Schweiz. the sections Respecting rules for retrodeformable cross greatly Bt RKHARD. M. 1988: L'Helvétique de la bordure occidentale du massif de reduces the interpretational leeway. It permits the construction l'Aar (évolution tectonique et métamorphique). Eclogae geol. Helv 81/1. of a plausible structure for the Drusberg and the Axen 63-114. 1990: Aspects of the large scale miocene deformation in the most external nappe. Although there is still some interpretation required, the part of the Swiss (Subalpine Molasse to Jura fold belt). Eclogae geol. cross sections unveil new connections by putting old problems Helv. 83/3.559-583. into a larger scale context. 1999: Strukturgeologie im Bereich Alp Transit. Tagungsband zum Symposium Synsedimentary normal faults that were active during the Geologie Alp Transit. 15.-17. Feb. 1999 in Zürich. A.A. Balkema. Cretaceous and Tertiary leave their mark also in the older Rotterdam. 45-56. Collet. L.W. & Paréjas. E. 1928: Carte géologique de la Chaine de Ia sediments and can therefore be used to determine the initial Jungfrau. Carte spéciale No. 113 (1:25 000). Comm. géol. Suisse. position of the Drusberg nappe relative to the Axen nappe. 1931: Géologie de la Chaine de la Jungfrau. Matér. Carte géol. Suisse, The retrodeformed sections show that the Burgle fault must [n.s.] 63.

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The Helvetic nappes in the Bernese Oberland 171 Plate I Section I: Reichenbach-Breithorn h36 60 Section III: Hengst-Urbachsattel ¦J I 000 Bielen Wandelgrat Rosenlaui Urbachsaltel 618.420 633.250 Steingrat Hiirnhegg Hengst Bnefengrat Mittl. Stafel Kienhol/ [65.000 147.500 Erlibach Dreispitz Aabeberg ZahmAndrist Hundshorn But lasse Nollen Wild Andnst Gspaltenhorn

Fig -ib Sei renllv Fig. 3a

Bachli-Giesenen Fault

VvsAw Niesen n

Aabeberg-f. Burgle-f

Iectonic overview Iectonic overview

Bundstock element (Axen-n.) Drusberg napp r~^~] Doldenhorn nappe Okm Drusberg rr.ipp Subalpine Flysch ~~J Parautochthonous sl Axen nappe Gellihorn nappe "~1 Autochthonous H 4xen nappe ^J Subalpine Molasse WEM Autochthonous J I

and Section Autochthonous Drusberg II Linden-Jungfraujoch Axen nappe 643.650 USME2 Jungtraujoch 153.100 D Spitzi Flue Niedertiom Darliggrat Tertiary USME1-A2

Lake Thun Wang-Fm. USM Al Garschella-Fm. JD i 616740 Seewen-Fm. 192 .Slid Hilfern ihrusl UMM Amden-Fm. Schrattenkalk-Fm. I nun-1 n Mesozoic Schwalmera ls. Linden-1 620.220 617.740 Blueme thrusl 178.800 -loheunt.Sundlauenen talli! 188567 Triassic Drusberg-Fm. 7», r OMM/OSM Kieselkalk-Fm. Crystalline Sichel ls. Doldenhorn nappe Diphyoides ls. V-iTlLTl Jjlllll Gellihorn nappe Palfris-Fm. Oehrli marls Tertiary Quinten ls. Kieselkalk-Fm. Erzegg-Fm. Öhrli ls. Senilism. Betlis ls. Tectonic overview Hochstollen-Fm. Öhrli marls r> - _, permo-carboniferous Quinten ls. Glockhaus-Fm. ¦•. * 345° 317 Schilt-Fm. I Triassic- Mols-Fm. Doldenhorn | Molasse Drusberg nappe j nappe Subalpine I m.Jurassic Axen nappe H Au loc hthonous Crystalline "Lias"

Hänni & O. A. Pfiffner The Helvetic nappes in the Bernese Oberland